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This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 1, 2002.
Copyright (C) The Internet Society (2001). All Rights Reserved.
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This document discusses a method used by the Linux FreeS/WAN project to opportunistically encrypt traffic between peers without specific pre-arrangement. It describes the infrastructure necessary to support this configuration. Opportunistic Encryption (OE) provides major simplifications of typical configurations, as it scales linearly rather than quadratically in the number of systems involved.
There are naturally some risks, which are described.
This document is offered up as an Informational RFC.
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The Linux FreeS/WAN project started in 1996. Its goal was to secure 5% of the Internet traffic against passive wire-tapping, increasing over time to 80%.
The Internet Architecture Board (IAB) and Internet Engineering Steering Group have taken a strong stand that the Internet should use powerful encryption to provide security and privacy. This project attempts to put this policy into practice by providing practical means to implement them.
This idea does not originate with this project; lots of people have been working on it for years. The encryption protocols for these boxes are called IPsec (IP Security). They have been developed by the IP Security Working Group of the Internet Engineering Task Force, and became a standard in 1999. See RFC 2401[2].
The project believes that these protocols are the best chance to do that, because they can be deployed very easily, without changing hardware or software or retraining of users.
The use of "opportunistic encryption" offers the "fax effect". As each person installs one for their own use, it becomes more valuable for their neighbors to install one too, because there's one more person to use it with. The software automatically notices each newly installed box, and doesn't require a network administrator to reconfigure it.
This document describes the infrastructure needed to support this effort.
The term S/WAN is a trademark of RSA Data Systems, and is used with permission by this project.
To aid in understand the relationship between security processing and IPsec we divide network traffic into four categories:
- deny
- networks to which traffic is always forbidden
- permit
- networks to which traffic in the clear is desired
- opportunistic tunnel
- networks to which encryption should be done if possible, but sent in the clear otherwise
- configured tunnel
- networks to which encryption must be done, and never sent in the clear
This document describes the third category. The first two categories are provided by traditional firewall devices. The fourth category is presently the offering of many Virtual Private Network (VPN) devices.
Category one, denied traffic by IP address, requires no authentication.
Category two, permitted traffic by IP address, requires no authentication. This is the normal default on the Internet.
Category four, encrypt traffic or drop it, requires authentication of the end points. As the number of end points is typically bound and is typically under the authority of a single authority, arranging for exchange of authentication material, while difficult, does not require any new technology. The mechanism described here provide an additional way to provide the authentication materials, notably a public key method that does not require deployment of an X.509 based infrastructure.
Opportunistic encryption involves creating tunnels with other nodes with are essentially strangers. This is done without any prior bilateral arrangement. There is therefore the difficult question of how does one know who one is talking to.
One answer is that one does not know who one is talking to. No useful authentication can be done, so do not even try. This mode of operation has been given the name "anonymous encryption".
Although a useful mode, it is not the goal of this project. It is a useful starting point, but the system should permit additional layers of trust to be built upon this system. In the described system, the anonymous encryption case is what results without DNSSEC. Were anonymous encryption the end goal, simpler methods are available to achieve this goal.
However, an essential premise of building private connections with strangers is that packets received through these opportunist tunnels packets are no more special than packets that arrived in the clear.
Unlike in a VPN scenario, these packets should not be given any special exceptions when it comes to auditing, further authentication or firewalling.
On the outbound side, when initiating opportunistic encryption, it becomes a local matter what to do if one fails to setup a tunnel. It may be that the packet goes out in the clear, or it may be dropped. This is a local configuration matter.
In sum, we gain wider privacy (for the Internet at large) at minimal cost: the cost is the need to reassess assumptions about the relationship between IPsec authentication and further local access control.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [1]
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The following network diagram is used in the rest of this document as the canonical diagram
[Q] [R] AS2 . . [A]----+----[SG-A]...+....+....[SG-B]-------[B] AS1 | . PI . [D]----+----[SG-D]...+ +....[C] AS3
In this diagram, there are four end-nodes: A, B, C and D. There are three gateways, SG-A, SG-B, SG-D. A, D, SG-A and SG-D are part of the same administrative authority. SG-A and SG-D are on two different exit paths from organization 1. SG-B/B is an independent organizations. Nodes Q and R are nodes that are on the Internet. PI is the Public Internet ("The Wild").
The following terminology is used in this document:
- security gateway:
- a system that performs IPsec tunnel mode encapsulation/decapsulation. [Gx] in the diagram
- Alice:
- node [A] in the diagram. When an IP address is needed, this is 192.1.0.65.
- Bob:
- node [B] in the diagram. When an IP address is needed, this is 192.2.0.66.
- Carol:
- node [C] in the diagram. When an IP address is needed, this is 192.1.1.67.
- Dave:
- node [D] in the diagram. When an IP address is needed, this is 192.3.0.68
- SG-A
- Alice's security gateway. Internally it is 192.1.0.1, externally it is 192.1.1.4.
- SG-B
- Bob's security gateway. Internally it is 192.1.2.1, externally it is 192.1.1.5.
- SG-D
- Dave's security gateway. Also Alice's backup security gateway. Internally it is 192.3.0.1, externally it is 192.1.1.6.
- -
- a single dash represents clear-text packets
- =
- an equals sign represents phase 2 (IPsec) cipher-text packets
- #
- a hash sign represents phase 1 (IKE) cipher-text packets
- .
- a period represents an untrusted network of unknown type
- configured tunnel:
- Contrast with opportunistic tunnel. A tunnel that is directly/deliberately/hand configured on participating gateways. Configured tunnel are typically given a higher level of trust than opportunistic tunnels.
- road warrior tunnel:
- a configured tunnel connecting one node with a fixed IP address and one node with a variable IP address. A road warrior (RW) connection must be initiated by the variable node, since the fixed node does not know what the address for the "road warrior" currently is.
- phase 1 SA
- an ISAKMP/IKE security association
- phase 2 SA
- an IPsec security association
- tunnel
- another term for a set of phase 2 SA
- NAT
- Network Address Translation (see [10])
- NAPT
- Network Address and Port Translation (see [10])
- anonymous encryption:
- The process of encrypting a session without any knowledge of who the receiving party is. That is, no authentication of identities is done.
- opportunistic encryption:
- The process of encrypting a session with knowledge of who the receiving party is.
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Alice Gate1 DNS Gate2 Bob (1) ------(2)--------------> <-----(3)--------------- (4)----(5)----->----------(6)------>------(7)-----> <----(10)-----<----------(9)------<------(8)------ (11)----------->----------(12)----->--------------> <-------------<-------------------<---------------
Alice wants to send a packet to Bob, say a ping packet. Without the presence of opportunistic encryptors, the following events occur:
- (1)
- Human or application 'clicks' with a name
- (2)
- Application looks up name in DNS to get IP address
- (3)
- Resolver returns A record to application
- (4)
- Application starts a TCP session or UDP session, OS sends packet
- (5)
- Packet is seen at first gateway from Alice (SG-A)
- (6)
- Packet is seen at last gateway before Bob (SG-B)
- (7)
- First packet from Alice is seen by Bob
- (8)
- First return packet is sent by Bob
- (9)
- Packet is seen at Bob's gateway
- (10)
- Packet is seen at Alice's gateway
- (11)
- OS hands packet to application, Alice sends another packet
- (12)
- a second packet traverses the Internet
In the presence of properly configured opportunistic encryptors, the event list is extended.
Alice SGate1 DNS SGate2 Bob (1) ------(2)--------------> <-----(3)--------------- (4)----(5)----->+ ----(5B)-> <---(5C)-- -------------(5D)---> <------------(5E1)--- #############(5E2)##> <############(5E3)### #############(5E4)##> <############(5E5)### <----(5F1)-- -----(5F2)-> #############(5G1)##> <----(5G2)-- -----(5G3)-> <############(5G4)### ============(6)====>------(7)-----> <-----(10)----<==========(9)======<------(8)------ (11)----------->==========(12)=====>--------------> <-------------<===================<---------------
- (1)
- Human or application clicks with a name
- (2)
- Application initiates DNS mapping.
- (3)
- resolver returns A record to application
- (4)
- Application starts a TCP session or UDP
- (5)
- SG (host or SG) sees packet to target, buffers it.
- (5B)
- SG asks the DNS for TXT record
- (5C)
- DNS returns TXT record(s)
- (5D)
- Initial IKE Main Mode Packet goes out
- (5E)
- IKE ISAKMP phase 1 succeeds
- (5F)
- SG-B asks the DNS for TXT record to prove SG-A agent for Alice
- (5G)
- IKE phase 2 negotiation
- (5H)
- IPsec succeeds, sets up tunnel for communication between Alice and Bob
- (6)
- buffered packet is sent by SG-A
- (7)
- packet is received by SG-B, and decrypted, sent to Bob
- (8)
- Bob replies, packet is seen by SG-B
- (9)
- SG-B already has tunnel up with SG-A, uses it
- (10)
- SG-A decrypts packet and give it to Alice
- (11)
- Alice receives packet. Sends new packet to Bob
- (12)
- SG-A gets second packet, sees that tunnel is up, uses it
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For the purposes of this section, we will describe on the changes that occur between Figure 2 and Figure 3. This means time points 5, 6, 7, 9 and 10.
At point (5), the Security Gateway intercepts the packet, as it would with any configured tunnel. The packet is buffered. If there is already a buffered packet that matches the same selectors, the new packet is dropped. There are no differences from regular IPsec processing here.
At the IPsec SPD level, opportunistic encryption may be considered as being simply a per-host keyed (see section 4.4.1) tunnel to 0.0.0.0/0.
Should the prospective responder not have OE configured (or: not respond to attempts to establish a phase 1 SA), the initiator MAY create an SPD entry to permit clear-text packets to this machine. This is a local policy decision. In the configured tunnel situation the packet would never be sent in the clear.
The IKE daemon, having been informed of the need for a tunnel between Alice and Bob performs a DNS lookup. This DNS lookup serves two purposes:
A reverse lookup is done on the IP address of Bob, mapped into the in-addr.arpa in the usual way. The lookup requests TXT records. These are interpreted as described in Use of TXT record.
In this context, the returned record might contain:
X-IPsec-Server(10)=192.1.1.5 AQMM...3s1Q==with SG-B's IP address and public key listed.
Given the IP address of the gateway information, SG-A is now in a position to create a phase 1 SA with SG-B.
SG-A sends the initial ISAKMP message, with proposals.
SG-B receives the message and responds with its choice of proposals for the phase 1 SA.
SG-A sends a Diffie-Hellman exponent.
SG-B responds with a Diffie-Hellman exponent.
SG-A uses the phase 1 SA to send its identity under encryption. The choice of identity is discussed in Phase 1 IDs.
Security Gateway 2 asks DNS for the public key of the initiator. This is done by asking for a KEY record by IP address in the reverse map. That is, a KEY resource record is queried for 4.1.1.192.in-addr.arpa (recall that SG-A's external address is 192.1.1.4) The resulting public key is used to authenticate the initiator. See Use of KEY record for further details.
The format of the KEY record is described in Use of KEY record.
The format of the TXT record that is returned is described in Use of TXT record.
SG-B sends its ID along with authentication material.
Having established mutually agreeable authentications (via KEY) and authorizations (via TXT), SG-A proposes to create an IPsec tunnel for packets transiting from Alice to Bob. This tunnel is established for only the A/B combination, not for any subnets that may be behind SG-A and SG-B.
While the identity of the SG-A has been established, its authority to speak for Alice has not yet been confirmed. This is done by doing a reverse lookup on A's address for a TXT record. \
The returned key and IP address should match that of SG-A.
Should additional communication occur between, for instance, D and B via SG-A and SG-B, a new tunnel (phase 2 SA) would be established. The phase 1 SA may be reusable.
The packet which was saved at step (5) is sent through the newly created tunnel to SG-B. Bob receives it at (7) and replies at (8).
At (9), SG-B has already established an SPD entry mapping Bob->Alice via a tunnel, so this tunnel is simply applied. The packet is encrypted to SG-A, decrypted by SG-A and passed to Alice at (10).
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The IKE wire protocol needs no modifications. The major changes are implementation issues relating to how the proposals are interpreted, and from whom they may come.
As Opportunistic Encryption is designed to be useful between peers without prior operator configuration, an IKE daemon must be prepared to negotiate phase 1 SAs with any node. This may require a large amount of resources to maintain cookie state, as well as large amounts of entropy to for nonces, cookies, etc.
The major changes to support Opportunistic Encryption are at the IKE daemon level. These changes relate to handling of key acquisition requests, lookup of public keys and TXT records, and interactions with firewalls and other security facilities that may be coresident on the same gateway.
In a typical configured tunnel situation, the address of SG-B is provided via configuration. Furthermore, the mapping of SPD entry to gateway is typically a 1:1 mapping. When the 0.0.0.0/0 SPD entry technique is used, then the mapping to a gateway is determined by the reverse DNS records.
The need to do a DNS lookup and wait for a reply will typically introduce a new state and a new event source (DNS replies) to IKE. Although a synchronous DNS request can be done for proof of concept, this will not scale very well.
Use of an asychronous DNS lookup will also permit overlap of DNS lookups with protocol steps.
SG-A will have to establish its identity. This is done by use of an IPv4 ID in phase 1.
There are many situations where the administrator of SG-A may not be able to control their reverse DNS. Typical situations include dialup connections and most residential-type broadband Internet access (ADSL, cable-modem). In these situations, a fully qualified domain name which is under the control of SG-A's administrator may be used. The FQDN ID should be used in phase 1. See Use of FQDN IDs for more details and restrictions.
Upon receipt of a phase 1 SA proposal with either an IPv4 (IPv6) ID, or an FQDN ID, an IKE daemon will need to examine local caches and configuration files to determine if this is part of a configured tunnel. If none is found, then the implementation should attempt to retrieve a KEY record from the reverse DNS (in the case of an IPv4/IPv6 ID), or from the forward DNS in the case of FQDN ID.
It is reasonable that if other non-local sources of policy are used (COPS, LDAP, ...) that they be consulted concurrently, but that some clear ordering of policy be provided. Note that due to variances in latency, one must wait for positive or negative replies from all sources of policy before making any decisions.
The present implementation does not use DNSSEC to explicitly verify the authenticity of zone information, or use the NXT records to provide authentication of the absence of a TXT or KEY record. These are important considerations for practical use.
Main mode SHOULD be used.
Support for the second oakley (1024) is a MUST. Implementations SHOULD support for group 5 (1536).
For Phase 1, a proposal of 3DES-CBC with MD5 authentication SHOULD be used.
SG-A SHOULD use an ID_IPV4_ADDR, of the external interface of SG-A for phase 1. The authentication method should be RSA public key signatures. (There is an exception, see Use of FQDN IDs)
The RSA key for SG-A SHOULD be placed into a DNS KEY record in the reverse space of SG-A. (i.e. using in-addr.arpa.)
SG-A SHOULD propose a tunnel between Alice and Bob, using 3DES-CBC mode, MD5 or SHA1 authentication. Perfect Forward Secrecy SHOULD be specified.
Authorization for SG-A to act on Alice's behalf is determined by looking for a TXT record in the reverse map at Alice's address.
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In order to establish its own identity, SG-A and SG-B must publish their public keys in their reverse DNS. This is just done via DNSSEC's KEY record. See section 3 of RFC 2535[6].
For example:
KEY 0x4200 4 1 AQNJjkKlIk9...nYyUkKK8
- 0x4200
- The flag bits, indicating that this key is prohibited for use confidentiality (it authenticates the peer only, DH is used for confidentiality), and that this key is associated with the non-zone entity whose name is the RR owner name. No other flags are set.
- 4
- This indicates that this key is for use by IPsec
- 1
- An RSA key is present
- AQNJjkKlIk9...nYyUkKK8
- the public key of the host as described in [7]
A TXT record is published by Alice (Bob) to provide authorization for SG-A (SG-B) to act on their behalf. This record is located in the reverse DNS (in-addr.arpa) for them. The reverse DNS SHOULD be secured by DNSSEC, although this is presently not deployed yet.
If Alice's address is P.Q.R.S, then she can authorize another node to act on her behalf by publishing records at:
S.R.Q.P.in-addr.arpa
The resource record is expected to include a record that follows the following syntax, as suggested in [5]:
X-IPsec-Server(P)=A.B.C.D KEY
- P:
- the P specifies a precedence for this record. This is similar to MX record preferences. Lower numbers have stronger preference.
- A.B.C.D:
- specifies the IP address of the Security Gateway for this client machine.
- KEY:
- is the encoded RSA Public key of the Security Gateway. This is provided here to avoid a second DNS lookup.
It has been suggested to use the OPT or CERT records instead of a TXT record.
OPT resource records, as defined in [4] are not intended to be used for storage of information. They are not to be loaded, cached or forwarded. They are therefore inappropriate for use here.
Use of CERT records will be investigated in a future version of the specification.
Unfortunately, not every administrator has control over the contents of the reverse-map. The only case where we can work around this is where the initiator (SG-A) has no suitable reverse map. In this case, the authorization record present in the reverse map of Alice may refer to a FQDN instead of an IP address.
In this case, the client's TXT record gives the fully qualified domain name (FQDN) in place of its security gateway's IP address. The initiator should use the ID_FQDN ID-payload in phase 1. A forward lookup for a KEY record on the FQDN must yield the initiator's public key.
This method can also be used when the external address of SG-A is dynamic.
If SG-A is acting on behalf of Alice, then Alice must still delegate authority for SG-A to do so in her reverse map. When Alice is SG-A (i.e. Alice is acting as an end-node) then there is no need for this.
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There are no fundamentally new issues for getting opportunistic encryption to work in the presence of network address translation. Rather there are just the regular issues. There are several situations to consider for NAT:
If SG-A is also performing Network Address Translation on behalf of Alice, then the packet should be translated prior to being subjected to opportunistic encryption. This is in contrast to typical configured tunnel which often exist to bridge islands of private network address space. SG-A will use the translated source address for phase 2, and so SG-B will look that address up to confirm SG-A's authorization.
In the case of NAT, the reverse map MUST be under SG-A's administrator's control, so placing of TXT records will be possible.
In the case of NAPT, the address will be SG-A. The ability to get KEY and TXT records in place will again depend upon whether or not there is administrative control over the reverse map. While FQDN style can be used to get around part of the problem, the authorization for the FQDN to act upon SG-A's behalf can not be established, and so the typical residential case of NAPT being used to connect a household network can not be accommodated.
Should the appropriate administrative records be possible to provide, it will generally be impossible to provide for separate tunnels for Alice vs D that transit through SG-A.
If there is a NAT or NAPT between SG-A and SG-B, then normal IPsec NAT traversal rules would apply. In addition to the transport problem which can be solved via proposals like [3], there is the issue of what phase 1 and phase 2 IDs to use. While FQDN could be used during phase 1 for SG-A, an appropriate ID for phase 2 may not exist. Without this, there is nothing which would permit SG-B to determine that SG-A was in fact authorized to speak for Alice.
There is an unusual case where a public network exists behind SG-A, while SG-A itself is behind a NAT. This public network may in fact not be routed publically, but as it is unique, the reverse map may be consulted to provide appropriate authorization for SG-A.
If Bob is behind a NAT (perhaps SG-B), then there is in fact no way for Alice to address a packet to Bob. Not only is opportunistic encryption impossible, but it is also impossible for Alice to initiate any communication to Bob. It may be possible, if Alice has a public address, for Bob to initiate.
Opportunistic encryption is therefore not generally feasible from behind a NAT/NAPT.
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When Alice and SG-A are components of the same system, then this is considered to be a host implementation. The situation remains unchanged.
Components marked Alice are simply the upper layers (TCP, UDP, the application), and SG-A is the IP layer.
Note that tunnel mode is still recommended.
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If there are multiple paths between Alice and Bob (such as illustrated in the diagram with SG-D) then additional DNS records are required to establish authorization.
example to be provided.
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When to tear tunnels down is a bit problematic, but as an a potentially unbounded number of them may be created, a mechanism to remove them is required.
There are two methods in which the tunnel may be removed: by expiring or with explicit deletion. Deleting the tunnel requires that both ends support authenticated delete messages. This in turn requires that both ends are willing to maintain the phase 1 ISAKMP SA. If either end wishes to dispose of the ISAKMP SA, then the expiry method should be used instead, as there is little point in bringing up a second ISAKMP SA just to remove an unused phase 2 SA.
In the expiry method, the tunnel is simply allowed by the IKE daemon to expire rather than rekeying it.
Regardless of which method is used, a method is required to determine if the tunnel is still in use. This is a local matter, but the following criteria are what is used by the FreeSWAN project. This criteria is currently implemented in the key management daemon, but could also be implemented at the SPD layer where no polling would be required.
- 1
- a short initial (soft) lifespan of 1 minute is set. This is done since many net flows in fact last only a few seconds.
- 2
- upon expiry, a check is made to see if the tunnel was used by traffic in either direction during the last half of this period. If so, assign a longer tentative lifespan, of 20 minutes, after which, look again. If not, close the tunnel.
The tentative lifespan is independent of rekeying; it is just the time when the tunnel's future is next considered. This should happen reasonably frequently, unlike rekeying, which is costly and shouldn't be too frequent.
Multi-step backoff algorithms are not worth the trouble; connections are considered to be either very short lived, or long lived, so a poll every 20 minutes is not considered a problem.
An implementation that can add new timers to the forwarding path could simply make use of an inactivity timer, and notify the key management daemon.
If the security gateway and the client host are one and the same, tunnel teardown decisions might wish to pay attention to TCP connection status, as reported by the local TCP layer. A still-open TCP connection is almost a guarantee that more traffic is coming, while the demise of the only TCP connection through a tunnel is a strong hint that none is.
It is recommended that IKE keying channels are maintained as long as possible so that delete operations may be done.
Teardown should always be coordinated with the other end. This means interpreting and sending Delete notifications.
On receiving a Delete for the outbound SAs of a tunnel (or some subset of them), tear down the inbound ones too, and notify the other end with a Delete. Tunnels need to be considered as bidirectional entities, even though the low-level protocols don't think of them that way.
When the deletion is initiated locally, rather than as a response to a received Delete, send a Delete for (all) the inbound SAs of a tunnel. If no responding Delete is received for the outbound SAs, try re-sending the original Delete. Three tries spaced 10s apart seems a reasonable level of effort. (Indefinite persistence is not necessary; whether the other end isn't cooperating because it doesn't feel like it, or because it is down/disconnected/etc., the problem will eventually be cleared up by other means.)
After rekeying, transmission should switch to using the new SAs (ISAKMP or IPsec) immediately, and the old leftover SAs should be cleared out promptly (and Deletes sent) rather than waiting for them to expire. This reduces clutter and minimizes confusion.
Since there is only one keying channel per remote IP address, the question of whether a Delete notification has appeared on a ``suitable'' keying channel does not arise.
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If a DNS server fails to respond, then it is a local policy decision whether or not to permit communication in the clear. It should clear that mounting a denial of service attack on the DNS server responsible for a particular network's reverse map is an easy thing to do. Such an attack may cause all communication with that network to go in the clear. Please note that this is an active attack.
At the same time, there are still a very large number of networks that do not have properly configured reverse maps. Further, the effect of the above denial of service attack, if the policy is not to communicate, is that the target network becomes isolated. This is why this decision MUST be a matter of local policy.
In this situation, DNS records claim that opportunistic encryption should occur, but the target gateway either does not respond on port 500, or refuses the proposal. This may be due to a crash/reboot, due to misconfiguration, a firewall filtering port 500.
The receipt of ICMP port, host or network unreachable messages should be taken as a sign that there is a potential problem, but MUST NOT cause communication to fail immediately.
It is recommended that in these situations that a clear log be produced about the problem. A local policy should dictate if communication is then permitted in the clear at this point.
Tunnels sometimes go down because the other end crashes, or disconnects, or has a network link break, and there is no notice of this in the general case. (Even in the event of a crash and successful reboot, other SGs don't hear about it unless the rebooted SG has specific reason to talk to them immediately.) Over-quick response to temporary network out- ages is undesirable... but note that a tunnel can be torn down and then re-established without any user-visible effect except a pause in traffic, whereas if one end does reboot, the other end can't get packets to it at all (except via IKE) until the situation is noticed. So a bias toward quick response is appropriate, even at the cost of occasional false alarms.
A mechanism for recover after reboot is not specified in this document, as it is a topic of current research.
A deliberate shutdown should include an attempt to notify all other SGs currently connected by keying channels, using Deletes, that communication is about to fail. (Again, these will be taken as teardowns; attempts by the other SGs to negotiate new tunnels as replacements should be ignored at this point.) And when possible, SGs should attempt to preserve information about currently-connected SGs in non-volatile storage, so that after a crash, an Initial-Contact can be sent to previous partners to indicate loss of all previ- ously-established connections.
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The method of obtaining information is by reverse DNS lookup causes problems for people who can not control their reverse DNS bindings. This is an unresolved problem in this version, and is out of scope.
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Configured tunnels are those which are setup using bilateral mechanisms: exchanging public keys (raw RSA, DSA, PKIX), pre-shared secrets, or by referencing keys that are in known places (distinguished name from LDAP, DNS). These keys are then used to configure a specific tunnel.
A pre-configured tunnel may be on all the time, or may be keyed only when needed. The end points of the tunnel are not necessarily static: many mobile applications ("road warrior") are considered to be configured tunnels.
The key consideration is that configured tunnels are assigned specific security properties. They may be trusted in different ways. This is usually related to exceptions to firewall rules, exceptions to NAT processing, and to bandwidth or other quality of service restrictions.
Opportunistic tunnels are not inheirently trusted in anyway. They are created without prior arrangement. As the two parties are strangers, there MUST be no confusion of packets that arrive from opportunistic peers and those that arrive from configured tunnels. A security gateway MUST take care that an opportunistic peer can not impersonate a configured peer.
It is recommended that an implementation SHOULD prevent opportunistic peer from impersonating another opportunistic peer. This is equivalent to inforcing ingress filtering of source addresses.
An implementation suggestion is to logically treat opportunistically tunnel packets as if they arrive on a distinct logical interface from other configured tunnels. As the number of opportunistic tunnels that may be created automatically on a system is potentially very high, careful attention to scaling should be taken into account.
Typical usage of per-packet access control lists is to implement various kinds of security gateways. These are typically called "firewalls".
Typical usage of virtual private network (VPN) within a firewall is to bypass all or part of the access controls between two networks. Additional trust (as outlined in the previous section) is given to packets that arrive in the VPN.
Packets that arrive via opportunistically configured tunnels MUST not be trusted. Any security policy that would apply to a packet arriving in the clear SHOULD also be applied to packets arriving opportunistically.
There are several different forms of denial of service which an implementor should concern themselves with. Most of these problems are shared with security gateways that have large numbers of mobile peers (road warriors).
The design of ISAKMP/IKE, and its use of cookies defend against many kinds of denial of service. There is an assumption that if the phase 1 (ISAKMP) SA is authenticated, that it was worthwhile creating. Opportunism changes this assumption. As the gateway will communicate with anyone, it is possible to form phase 1 SAs with any machine on the Internet.
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There are no known numbers which IANA will need to manage.
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Michael C. Richardson | |
Sandelman Software Works | |
152 Rochester Street | |
Ottawa, ON K1R 7M4 | |
CA | |
EMail: | mcr@sandelman.ottawa.on.ca |
URI: | http://www.sandelman.ottawa.on.ca/ |
D. Hugh Redelmeier | |
Mimosa | |
Somewhere | |
Toronto, ON | |
CA | |
EMail: | hugh@mimosa.com |
URI: | http://www.sandelman.ottawa.on.ca/ |
Henry Spencer | |
SP Systems | |
Box 280 Station A | |
Toronto, ON M5W 1B2 | |
Canada | |
EMail: | henry@spsystems.net |
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